In recent times, the increased emphasis on environmental pollution problems has affected all areas of society. In the coatings industry, this emphasis has taken the form of a major effort to eliminate or substantially reduce organic solvent emissions from coating compositions. This has led to an increasing interest on the part of the coatings industry in water-based coatings in which organic solvents are eliminated or at least greatly reduced. In even more recent times, this interest in water-based coatings has received added impetus as a result of the energy shortage, particularly the decline in U.S. oil production and the high cost of imported oil.
Water-based coating compositions derived from interpolymers of substituted, carboxylic acid amides, alpha, beta-ethylenically unsaturated carboxylic acids, and other ethylenically unsaturated monomers are known in the art, e.g., see U.S. Pat. No. 3,247,139 issued Apr. 19, 1966, and Example XV of U.S. Pat. No. 3,079,434, issued Feb. 26, 1963. However, the water-based coating compositions of the aforementioned patents differ materially in composition from the compositions of this invention in that the compositions described in the aforementioned patents do not contain a polyol component. Moreover, while the compositions mentioned in these patents are useful in certain applications they have been found to exhibit a number of serious disadvantages which materially limit their usefulness. Thus, the compositions of the aforementioned patents and, for that matter, water-based coatings in general, have been found to be very susceptible to solvent or water popping and sagging, or "curtaining", particularly when relatively thick films are deposited therefrom. Solvent or water popping manifests itself in the form of bubbles or pinholes in the cured film surface. Water-based coating compositions such as those described in the above-mentioned patents also tend to have lower than desired water and humidity resistance.
The exact cause of solvent or water popping is not known with centitude, but two theories have been advanced. One theory is that the film sets up structurally, or actually begins to crosslink before the last portion of the solvent or water is eliminated. This residual solvent or water cannot evaporate through the tough surface film and collects in tiny bubbles which may or may not rupture, depending upon the curing conditions.
Another theory is that as the resin cures, the water and/or alkanol given off during the crosslinking cure process is actually entrained under the film surface in the form of tiny bubbles.
Sagging or "curtaining" occurs when relatively thick films of the coating composition are applied to other than horizontal surfaces and is due to gravitational flow of the film, and/or to film resoftening during the curing period. In the coatings art, the term "sagging" denotes the tendency of a film to drain or flow from a non-horizontal surface in an uneven manner, while the term "curtaining" denotes the tendency of the film to drain or flow from said surface in a smooth, continuous manner.
Very recently, water-based coating compositions comprising methylolated amide interpolymers of high acid content and low molecular weight polyhydric alcohols have been disclosed in U.S. Pat. No. 3,860,549 to Sekmakas, issued Jan. 14, 1975. However, the compositions of the aforementioned patent are limited specifically to interpolymers derived from methylolated amides, which are unetherified, and thus differ materially from the compositions of the invention herein. The compositions of the aforementioned patent also exhibit serious disadvantages. Thus, compositions derived solely from methylolated amide interpolymers tend to exhibit short potlife and be prone to gellation.
In accordance with this invention, a water-based coating composition is provided which overcomes substantially all of the disadvantages referred to above. Thus, the water-based coating composition of the invention is one in which solvent popping or sagging is eliminated or at least substantially reduced. Moreover, since the compositions of the invention are based on n-alkoxyalkyl-substituted amides (i.e., etherified amides) they exhibit excellent stability.